P Klos-Wojtczak

Orexinergic theta rhythm in the rat hippocampal formation: In vitro and in vivo findings

Previous in vivo data suggested that orexin neuropeptides (ORXA and ORXB) synthetized in hypothalamic neurons were involved in the mechanism of generation of the hippocampal formation theta rhythm. Surprisingly, this suggestion has never been directly proved by experiments using intraseptal or intrahippocampal administration of orexins. In this study, involving the use of in vitro hippocampal formation slices and in vivo model of anesthetized rat, we provide the first convergent electropharmacological evidence that in the presence of both ORXA and ORXB the hippocampal formation neuronal network is capable of producing oscillations in the theta band. This effect of orexin peptides was antagonized by selective blockers of orexin receptors (OX1R and OX2R), SB 334867 and TCS OX2 29, respectively. These results provide evidence for a novel, orexinergic mechanism responsible for the production of theta rhythm in the hippocampal formation neuronal network.

The electrical coupling and the hippocampal formation theta rhythm in rats

Gap junctions (GJs) were discovered more than five decades ago, and since that time enormous strides have been made in understanding their structure and function. Despite the voluminous literature concerning the function of GJs, the involvement of these membrane structures in the central mechanisms underlying oscillations and synchrony in the neuronal network is still a matter of intensive debate. This review summarizes what is known concerning the involvement of GJs as electrical synapses in mechanisms underlying the generation of theta band oscillations. The first part of the chapter discusses the role of GJs in mechanisms of oscillations and synchrony. Following this, in vitro, ex vivo, and in vivo experiments concerning the involvement of GJs in the generation of hippocampal formation theta in rats are reviewed.

Vagus nerve stimulation produces a hippocampal formation theta rhythm in anesthetized rats

Vagus nerve stimulation (VNS) has been used for years to treat patients with drug-resistant epilepsy. In the present study, the effect of different stimulation protocols of VNS on the hippocampal formation (HPC) type II theta field potentials were evaluated in anesthetized rats. The following theta parameters were analysed: power, frequency and duration of theta epochs. We documented for the first time the presence of HPC type II theta in response to the application of VNS. A VNS-induced theta rhythm appeared in different experimental protocols and, depending on the current intensity, could occur directly during VNS (brief effect) or after vagal stimulation (delayed effect), using lower intensity stimuli.

Cell discharge correlates of posterior hypothalamic theta rhythm recorded in anesthetized rats and brain slices

Kowalczyk et al. (Hippocampus 2014; 24:7‐20) were probably the first to conduct a systemic study of posterior hypothalamic area (PHa) theta rhythm in anesthetized rats. They demonstrated that local PHa theta field potentials were tail‐pinch resistant and could be generated in urethane‐anesthetized rats independently of ongoing hippocampal formation theta rhythm. These in vivo data were also confirmed in PHa slice preparations perfused with cholinergic agonist, carbachol. In the current experiments we extend our earlier observations concerning PHa theta rhythm. Specifically, PHa field potentials were analyzed in relation to the ongoing local cell firing repertoire. Single‐unit discharge patterns of cells localized in the posterior hypothalamic and supramammillary nuclei were characterized according to the criteria that was developed previously to classify theta‐related cells in the hippocampal formation. The present study demonstrated that in addition to the earlier described theta‐related cells (theta‐on, theta‐off and gating cells) the PHa also contains cells discharging in a very regular manner, which were labelled “timing cells”. This type of neuron has not been previously documented. We suggest that “timing cells” form a part of the ascending brainstem synchronizing pathway, provideing a regular rhythmic signal which facilitates the transduction of tonic discharges of cells localized in the brain stem into theta‐frequency rhythmic discharges.

Cell discharge correlates of posterior hypothalamic theta rhythm. Recipe for success in recording stable field potential.

The theta rhythm discovered in the posterior hypothalamus area (PHa) differs from theta observed in the hippocampal formation. In comparison to hippocampal spontaneous theta, the theta recorded in the PHa is rarely registered, has lower amplitude, often disappears, and sometimes returns after a few minutes. These features indicate that spontaneous theta recorded in the PHa is not an appropriate experimental model to search for the correlation between PHa cell discharges and local field potential. In this paper we present standard experimental conditions necessary to record theta-related cells in the PHa in anesthetized rats. Three pharmacological agents were used in the experiments to induce PHa theta rhythm in urethanized rats: carbachol (CCH), carbenoxolone and kainic acid, which are potent enough to induce well-synchronized PHa theta. However, CCH was found to be the best pharmacological tool to induce PHa theta oscillations, due to its longest duration of action and lack of preliminary epileptogenic effects. It seems that CCH-induced theta can be the most suitable pharmacological model for experiments with the use of protocol of long-lasting recordings of PHa theta-related cell discharges.

Some technical issues of vagal nerve stimulation. An approach using a hippocampal formation theta rhythm

Previously, we have demonstrated that hippocampal (HPC) theta rhythm can be produced, depending on current intensity, directly during vagal nerve stimulation (VNS) or with a time delay following stimulation. This suggests that theta EEG pattern can also be used as a bio-indicator of the efficiency of VNS. In the present study, we focused on three specific, technical issues related to the stimulation procedure of the vagal nerve: i/does the type of the electrode used for VNS and the technique of its implantation affect the parameters of the HPC theta rhythm? ii/does the type of electrode used determine the current intensity threshold of VNS-induced HPC theta? iii/is the repeatability of the VNS effect determined by the type of electrode used? We demonstrated that a platinum-iridium cuff electrode offers some important advantages over a tungsten electrode. Firstly, despite some possible mechanical and compression nerve damage related to permanent contact with the vagal nerve, it offers a lower current intensity threshold for inducing theta oscillations. Secondly, and most importantly, the cuff electrode offers repeatability of the VNS effect on the HPC theta rhythm. However, one disadvantage of using this type of an electrode is that the permanent pressure on the vagal nerve by the cuff itself may decrease the amplitude of the investigated field potential.

Is electrical coupling involved in the generation of posterior hypothalamic theta rhythm

Data obtained in in vitro experiments and urethane anaesthetized animals have revealed that the mechanisms responsible for the generation of hippocampal cholinergic theta rhythm are specifically affected by the administration of broad spectrum gap junctions (GJs) blocker – carbenoxolone (CBX). The aim of this study was to examine the effect of GJs modulation on the production of posterior hypothalamic theta. Specifically, we were interested in evaluating whether CBX could attenuate the theta rhythm recorded from the supramammillary nucleus and posterior hypothalamic nuclei, in both in vitro and in vivo preparations. The data we obtained from in vitro and in vivo preparations demonstrated that the administration of CBX did not suppress cholinergically induced theta in posterior hypothalamic area (PHa) slices nor the theta rhythm observed in the PHa of urethane anaesthetized rats. Moreover, the application of trimethylamine, while very effective in the enhancement of hippocampal theta rhythm, did not produce any changes in theta oscillations observed in either in vitro or in vivo posterior hypothalamic area preparations. These data show that electrical coupling via GJs is not involved in theta rhythm generation in the PHa. Surprisingly, we observed a significant enhancement of theta activity in response to the carbenoxolone administration in both in vitro and in vivo PHa preparations. The theta rhythm enhancement detected in those experiments was attenuated by the application of spironolactone (mineralocorticoid receptors antagonist). We suggest that the observed excitatory effects of CBX on posterior hypothalamic oscillatory activity in the theta band could be mediated by mineralocorticoid receptors.